This invention relates to security systems and more specifically to systems that can detect tunneling or other subsurface activity.
In many instances, it is desirable to secure a boundary. One aspect of securing a boundary is surveillance, which entails identifying when people or objects cross the boundary so that unauthorized movement across the boundary can be prevented. The boundary could be a national border, in which case surveillance may be used to prevent smuggling or illegal immigration. As another example, the boundary could be the perimeter of a secured facility, in which case surveillance may be used to prevent thieves or terrorists from gaining access to the facility. Though, boundary surveillance may also be used to prevent people from leaving a facility, such as a prison.
Various approaches are known for providing surveillance along a boundary. A traditional approach has been to deploy people along the boundary to observe activity. Electronic monitoring has also been employed, including with video cameras or heat detectors.
However, surveillance techniques that involve observing someone or something crossing the boundary can be thwarted by tunneling below the surface of the boundary. To detect such subsurface activity, systems have been developed in which vibration sensors are deployed along the boundary. The outputs of the vibration sensors are taken as an indication of underground activity and can therefore be used to detect the digging of tunnels or activity within tunnels, which indicates secretive attempts to cross the boundary. However, existing systems have a high false alarm rate from acoustic, RF and surface events.